1. Field of the Invention
The present invention relates to a Raman amplification method used in optical communication system and an optical transmission method using such a method.
2. Related Background Art
A frequency component (wavelength component) of an optical signal intensity of which is modulated and which has been used in existing optical communication system has a certain width. On the other hand, an optical fiber has a dispersion property that a propagating speed is changed depending upon a wavelength. Due to these two properties, when the optical signal is propagated through the optical fiber, a signal wave form is distorted because of difference in propagating speed between the wavelength components. When a pulse is inputted as the optical signal, since a pulse width is widened after propagation, this phenomenon is called as “pulse broadening due to dispersion” (for example, refer to Foundation Of Optical Waveguide written by Katsunari Okamoto and published From Corona Co.).
Although digital format is resistant to the change of wave form than analog format, error in detection is considerably increased if the overlap with adjacent bits become greater due to pulse broadening. To avoid this, in the prior arts, a wavelength having smaller (near zero) dispersion has been used to suppress the pulse broadening or spread pulse has been returned to the original form by delaying the wavelength component preceding through medium having dispersion opposite to that of the transmission line and by hastening the delayed wavelength component.
However, in the recent optical communication system, due to high output of the optical signal and multiplexing of the wavelength, a non-linear phenomenon within the optical fiber has become noticeable and distortion of the wave form could not be coped with only in the view point of dispersion. Main non-linear phenomena in question include self phase modulation (SPM), cross phase modulation (XPM) and four wave mixing (FWM). In SPM and XPM, phase of light is changed by a little change in refractive index of the optical fiber caused in accordance with light intensity. Since the change in phase causes instantaneous change in frequency and the changed amount is not constant, non-reversible wave form distortion is generated by the dispersion property of the optical fiber. The FWM is a phenomenon in which, when a polarization field is induced by plural inputted lights having different frequency, components different from the frequencies of the inputted lights are created, thereby generating light having new frequency. The FWM becomes noticeable particularly when the dispersion is near zero. If the light generated by the FWM coincides with the wavelength used as the signal, the error in detection will be increased.
As means for preventing deterioration of the transmission property based on such non-linearity of the optical fiber, there are two approaches. First approach is a method for reducing light intensity within the optical fiber to decrease the non-linear effect, and a second approach is a method for using a transmission method utilizing the non-linear effect. The former method can be realized by merely lowering an input level to the optical fiber or by utilizing an optical fiber having a large mode field diameter. The latter method can be realized by utilizing optical soliton. However, even when these methods are used, there remain the following problems.
If the input level to the optical fiber is lowered, since a signal/noise ratio (S/N ratio) at a receiving side is decreased, the error in detection will be increased. This can be interpreted so that a transmittable distance becomes shorter. Since the optical fiber having the large mode field diameter has a large dispersion slope (wavelength dependency of dispersion), it is difficult to set optimum dispersion with respect to all of channels in which the wavelengths are multiplexed. In the optical soliton communication system, due to perturbation (such as transmission loss or unevenness of dispersion) existing in the actual transmission line, dispersive wave out of the soliton condition are generated, which deteriorate the transmission property. As mentioned above, although the existing optical communication systems must be designed in careful consideration of several limitation factors, if there is no loss in the optical fiber as the optical transmission line, such limitations will be greatly relaxed. For example, in the transmission line having no loss, since there is no deterioration of the SIN ratio based on the propagation loss, the limitation factors caused by lowering the input level to the optical fiber are relaxed. Further, when the transmission line having no loss is applied to the optical soliton system, generation of the dispersive wave is greatly reduced. As one of conventional optical transmission lines most approaching to the transmission line having no loss, there is an optical transmission line in which loss is compensated by a Raman amplification.
A Raman amplification method utilizing Raman scattering of an optical fiber has advantages that the optical transmission line itself becomes an amplifier fiber and that any wavelength band can be amplified. In case of a silica-based optical fiber, peak of gain is generated at a long wavelength side greater than a wavelength of a pump light, i.e., in a frequency band having smaller frequency (than that of pump light) by about 13 THz. For example, 13 THz is a difference between wavelengths of 1450 nm and 1547 nm. Wavelength difference or frequency difference between the pump light and the gain peak is called as “Raman shift” which is a value depending upon composition of the optical fiber.
In general, in the Raman amplification method for communication, as shown in FIG. 21, a backward pumping scheme in which the pump light and the optical signal are propagated in opposite directions is adopted. Since a mechanism for generating Raman gain is operated at very high speed, in a forward pumping scheme in which the pump light and the optical signal are propagated in the same directions, fluctuation of intensity of the pump light is overlapped with the signal wave form as it is, with the result that the transmission property is deteriorated greatly. This is also described in Japanese Patent Application Laid-open No. 9-318981.
FIGS. 22 to 27 show general properties of intensity distribution of the pump light and optical signal along a longitudinal direction within the amplifier fiber of the Raman amplification method utilizing the conventional backward pumping scheme (regarding calculating methods, refer to “Nonlinear Fiber Optics”, Chap. 8, written by G. P. Agrawal and published from Academic Press, “Applied Optics”, Vol. 11, pp. 2489-2494, written by R. G. Smith and published in 1972, and “J. Quantum Electron”, Vol. QE-14, pp. 347-352, written by J. Auyeung and A. Yariv and published in 1978).
As Raman amplifiers utilizing the Raman amplification method, there are a distributed type in which the optical transmission line is used as the amplifier fiber, and a lumped type in which the amplifier fiber is provided independently from the optical transmission line. In the following explanation, the distributed type will be described. However, also in the lumped type, since performance of the optical signal and pump light in the amplifier fiber can be expressed by the same formula, the same effect can be achieved, although parameter values are different.
FIG. 22 is a graph showing change in pump light power and change in optical signal power. In this graph, a curve a indicates the optical signal power when incident power of the pump light is 100 mW (curve {circle around (1)}), a curve b indicates the optical signal power when incident power of the pump light is 200 mW (curve {circle around (2)}), and a curve c indicates the optical signal power when incident power of the pump light is 300 mW (curve {circle around (3)}). In the graph, a curve d indicates the optical signal power when the pump light is not inputted. As apparent from the curve d in the graph, when the pump light is not inputted, the optical signal power is attenuated in proportion to a propagating distance. When the pump light having attenuation constant of 0.25 dB/km is inputted, the Raman amplification is generated, thereby increasing the optical signal power. An increased amount of power is Raman gain. As apparent from a relationship between the curves {circle around (1)} to {circle around (2)} and the curves a to c in the graph shown in FIG. 22, the magnitude of the Raman gain is substantially in proportion to the incident power of the pump light. Since the pump light is also attenuated in proportion to the propagating distance, in the vicinity of a signal input end, intensity of the pump light becomes small, and, thus, the Raman gain is also small. Accordingly, the optical signal is attenuated at positions near the input end, and, it is subjected to great gain in the vicinity of an output end (end on which the pump light is inputted: in this example, position spaced apart from the input end by 50 km). If the intensity of the optical signal is sufficiently small, since the attenuation of the pump light depends upon the propagation loss, the intensity distribution of the pump light along the longitudinal direction is determined reasonably by the attenuation constant of the amplifier fiber. The distribution of the Raman gain along the longitudinal direction (intensity distribution of the optical signal along the longitudinal direction) is determined in accordance with the intensity distribution of the pump light along the longitudinal direction.
FIG. 23 is a graph showing the optical signal power in a case where attenuation constant αs of the optical signal is changed when the incident power of the pump light is constant and attenuation constant αp thereof is also constant (0.25 dB/km). A curve {circle around (1)} in this graph indicates a case where the attenuation constant αs is 0.3 dB/km, a curve {circle around (2)} indicates a case where the attenuation constant αs is 0.25 dB/km, and a curve {circle around (3)} indicates a case where the attenuation constant αs is 0.2 dB/km. As apparent from the graph shown in FIG. 23, the intensity distribution of the optical signal along the longitudinal direction within the optical transmission line is varied with the attenuation constant αs of the optical signal.
FIG. 24 is a graph showing change in optical signal power in a case where attenuation constant ap of the pump light is changed when the incident power of the pump light is constant and the attenuation constant αs of the optical signal is constant. A curve a in this graph indicates the optical signal power when the attenuation constant αp of the pump light is 0.3 dB/km (curve a curve b indicates the optical signal power when the attenuation constant αp is 0.25 dB/km (curve {circle around (2)}), and a curve c indicates the optical signal power when the attenuation constant αp is 0.2 dB/km (curve {circle around (3)}). As apparent from the graph shown in FIG. 24, when the attenuation constant αp of the pump light is changed, since the intensity distribution of the pump light along the longitudinal direction within the optical transmission line is different, the intensity distribution of the optical signal in the same direction is changed.
FIG. 25 is a graph showing a relationship between the pump light power and the optical signal power when a length of the amplifier fiber is changed. In this case, the incident power of the pump light is constant and the attenuation constants of the optical signal and the pump light are selected to be the same. From this graph, it can be found that, when the length of the amplifier fiber is changed, since the intensity distribution of the pump light along the longitudinal direction within the optical fiber is different, the intensity distribution of the optical signal in the same direction is also changed. In the graph shown in FIG. 25, curves {circle around (1)} to {circle around (5)} indicate the pump light powers when the length of the amplifier fiber is 10 km, 20 km, 30 km, 40 km and 50 km, respectively, and curves a to e indicate the optical signals powers when the length of the amplifier fiber is 10 km, 20 km, 30 km, 40 km and 50 km, respectively.
FIG. 26 is a graph showing change in optical signal power when the incident power of the pump light is constant and the attenuation constants of the optical signal and pump light are the same and Raman gain coefficient gR of the optical signal is different. In this graph, a curve a indicates the optical signal power when the gain coefficient gR is ⅓×10−13 m/W, a curve b indicates the optical signal power when the gain coefficient gR is ⅔×10−13 m/W, and a curve c indicates the optical signal power when the gain coefficient gR is 1×10-13 m/W. As apparent from this graph, when the magnitude of the Raman gain coefficient gR is changed, the magnitude of the Raman gain generated is changed, and the intensity distribution of the optical signal along the longitudinal direction within the optical transmission line is changed.
FIG. 27 is a graph showing change in optical signal power when the forward pumping is effected by the pump light having the constant incident power and when the backward pumping is effected. In this graph, a curve a indicates the optical signal power in the forward pumping, and a curve b indicates the optical signal power in the backward pumping. The value gR shown here is a value when the pumping wavelength is 1 μm. As apparent from this graph, if the pumping scheme is different, since the intensity distribution of the pump light along the longitudinal direction within the optical transmission line is different, the distribution of the Raman gain generated is changed, and the distribution of the optical signal is also changed. Incidentally, in the graph, a curve {circle around (1)} indicates power in the forward pumping and a curve {circle around (2)} indicates power in the backward pumping.
As general performance of a noise property of the transmission system using the optical amplifier, the fact that the noise property is greatly deteriorated by signal loss before the optical amplification. Thus, as is in the optical amplifier, when the amplifying effect has distribution along the longitudinal direction of the optical fiber, the noise property is deteriorated by loss at a position near the input end of the amplifier. On the other hand, in case of the backward pumping scheme, since the intensity of the pump light becomes smaller at the input end of the optical signal due to the propagation loss of the amplification optical fiber, the amplifying action at the input end of the optical signal also becomes small. Accordingly, in the backward pumping scheme, the loss at the input end of the optical signal becomes relatively great, which may deteriorate the noise property of the amplifier. Thus, in order to construct a Raman amplifier having good noise property, it was conventional or customary to use an optical fiber having small loss (with respect to both optical signal and pump light) as small as possible and to use an optical fiber having short length as short as possible.
On the other hand, in case of the Raman amplifier of distributed type in which the optical transmission line is used as the amplifier fiber, a high S/N ratio must be maintained while suppressing the non-linear effect in the optical transmission line. To this end, it is ideal to obtain a condition of a non-loss transmission line in which a level of the optical signal along the longitudinal direction within the optical transmission line becomes constant, and, in this case, it is desirable that intensity of the pump light also becomes constant along the longitudinal direction of the optical transmission line. However, in the prior art, a distance by which this can be achieved was relatively short and such a distance was determined reasonably by parameters (fiber length, gain coefficient, attenuation constants of the pump light and optical signal) of the optical fiber constituting the optical transmission line. The reason is that the distribution of the pump light along the longitudinal direction of the optical transmission line could not be controlled.